Solar energy systems capture the radiant light and heat from the sun using a range of ever-evolving technologies such as solar heating, solar photovoltaics, solar thermal energy, solar architecture and most recently artificial photosynthesis.
Solar energy systems are an important source of renewable energy and its technologies are broadly characterized as either passive or active depending on the way they capture and distribute the solar energy or convert it into solar power. Active solar techniques include the use of photovoltaic systems, concentrated solar power and solar water heating to harness the energy. Passive solar techniques include orienting a building to the Sun, selecting materials with favorable thermal or light dispersing properties, and designing spaces that naturally circulate air. A hybrid system is one that combines both passive and active elements together into a single system.
One passive solar technique that is particularly useful are systems called, thermosiphons. A thermosiphon is a physical effect and refers to a method of passive heat exchange based on natural convection, in which a fluid circulates without the necessity of a mechanical pump. Thermosiphoning is used for circulation of liquids and volatile gases in heating and cooling applications, such as heat pumps, water heaters, boilers and furnaces. Thermosiphoning also occurs across air temperature gradients such as those utilized in a wood fire chimney, or solar chimney. However, a thermosiphon system for efficiently heating air up to a temperature that can be used for domestic hot water, as well as heating structures, has yet to be realized. Instead, air based thermosiphons are typically only for heating the air in buildings/structures and thermosiphons that heat water are used for domestic hot water.
It should be noted that thermosiphons can either be open-loop, such as when the substance in a holding tank is passed in one direction via a heated transfer tube mounted at the bottom of the tank to a distribution point or closed-loop circuits with return to the original container.
With respect to active systems, their use has increased dramatically over the past few decades and photovoltaic panels are now on roofs everywhere.
Unfortunately, the photovoltaic (PV) cells have not seen much of an increase in how efficiently they convert sunlight to electricity. Flat plate collectors use the same method for capturing thermal energy as originally designed many decades ago and the same is true for evacuated tube collectors as well.
One of the problems is that PV cells loose efficiency as the temperature of the panel increases.
For example, typical PV modules convert around 85% of incoming sunlight into heat. During peak conditions, this can result in a heat-generation of 850 W/m2 and PV module temperatures as high as 70° C. The electrical power produced by PV modules decreases linearly with increase in module temperature. For example, in bright sunlight, crystalline silicon PV modules may heat up to 20-30° C. above ambient temperature, resulting in a 10-15% reduction in power output relative to the rated power output for the PV module. Moreover, higher PV module temperatures may increase material degradation, such as thermal fatigue failure of interconnections between PV cells in the PV module.
The problem is that if the PV cells are cooled to a temperature of about 40° C. (104° F.), which is the point at which the efficiency really begins to drop, that the coolant medium used (air or liquid) does not have sufficient energy, by itself, to sufficiently heat a domestic hot water system.
Therefore, there continues to be a need for efficient ways boost the temperature of air that are either independent of other solar systems or part of a hybrid system.